The basis for the phenomenon of position-effect variegation in Drosophila melanogaster will be investigated using a cloned acid phosphatase-1 (Acph-1) gene as a molecular probe. The expression of the Acph-1 gene, when it has been relocated within or near broken heterochromatin, will be monitored at the level of mRNA production. In addition, the gene copy number in cells with polytene chromosomes will be measured to determine if reductions in mRNA are associated with lowered levels of genic DNA. These results will be compared to those obtained from cells with non-polytene chromosomes. The state of the chromatin adjacent to variegating and non-variegating Acph-1 genes will be analyzed with a series of probes which both surround and include the Acph-1 gene. This will be done in rearrangements which differ with regard to the distance between the breakpoint and the locus and/or the severity of the genic inactivation associated with the breakpoint. DNA spanning the heterochromatic-euchromatic junctions will be cloned, restriction mapped and sequenced in order to detect features common in those rearrangements which induce position effects. Finally, in transformation experiments the sequences and/or the amount of heterochromatin necessary to cause position effects will be determined using the Acph-1 gene as a model system. These experiments should help us understand the ways in which altered chromatin states affect gene activation as well as providing fundamental information about the causal basis of position effect variegation.